In an internal combustion engine, there is provided an air-fuel ratio control apparatus for detecting an oxygen concentration in exhaust by an oxygen concentration sensor provided at an exhaust system for reducing an uncombusted component in exhaust gas and controlling an air-fuel ratio of an air-fuel mixture to an engine to a target air-fuel ratio near to the stoichiometric air-fuel ratio by a feedback control in accordance with the detected oxygen concentration.
Further, in an exhaust system of an internal combustion engine, normally, a catalyzer using a three way catalyst is provided. The catalyzer is provided with a function of simultaneously reducing CO, HC, and NOx in exhaust gas at a near stoichiometric air-fuel ratio.
In the case of a vehicle having a light weight of a motorcycle or the like in which an engine is comparatively frequently used in a high rotational speed region or a high load region, it is known that an amount of NOx in exhaust gas is large. However, there poses a problem that a cleaning rate of NOx by a catalyzer in using an air-fuel ratio control apparatus of a background art having a system of converging an air-fuel ratio near to the stoichiometric air-fuel ratio, is very low in comparison with cleaning rates of other components of CO, HC and a reduction in NOx cannot sufficiently be achieved.